I have run an World Challenge Style FC at Shannonville numerous times with the CASC and EMRA sanctioning bodies (won overall in 1999 with the EMRA). The track is fun with little to hit if you go off course. You will find that you need different gearing in an RX7 during most of the track. I guess you could spend a ton on new diff ratios.
What track are you running, Fabi, Nelson, Pro Track or Long Track ? Another thing to watch out for is cooling. Much of the track is taken at over 6,000 in 2nd/ 4,000 in 3rd and if your cooling is questionable you could overheat in a few laps.